1. Field of the Invention
The present invention relates to a hydrostatic transmission having a hydraulic pump and a hydraulic motor fluidly connected with each other. Especially, it relates to a pressure regulation means for adjusting hydraulic pressure in a closed hydraulic circuit of the hydrostatic transmission.
2. Background Art
Conventionally, there is a well-known continuously variable transmission generally called a hydrostatic transmission or an HST which includes a hydraulic pump with a movable swash plate fluidly connected to a hydraulic motor with either a movable swash plate or a fixed swash plate, wherein the movable swash plate(s) is (are) moved so as to change the capacity of the hydraulic pump (the capacities of the hydraulic pump and motor), thereby changing the rotational speed and direction of rotational force output from the hydraulic motor. This transmission is used for various uses, e.g., for driving a vehicle.
Furthermore, as disclosed in Japanese Utility Model Provisional Publication No. 66454/92, there is a well-known HST provided with a neutral-returning spring and a shock absorber which are attached to a control arm interlockingly connected to the swash plate in the HST. By the neutral-returning spring, the movable swash plate is automatically returned to its neutral position when the control arm is released from an operational force.
The shock absorber prevents a dynamic brake which happens if the automatic returning of the movable swash plate to its neutral position is sudden.
On the other hand, Japanese Patent Provisional Publication No. 9385/98 points out a problem of the external arrangement of the shock absorber, wherein the movement of the shock absorber is unstable because of the influence of variable air conditions such as air temperature and the external shock absorber prevents the apparatus from being minimized. The document also proposes such a construction that, instead of the external shock absorber, pistons, which are pushed out by the hydraulic pressure of oil discharged from the hydraulic pump when a movable swash plate of the hydraulic pump is rotated from its neutral position, abut against the control arm. Such pistons are advantageous in their stable movement and contribution for minimization of the HST apparatus.
Furthermore, the document proposes means for adjusting the hydraulic pressure in the HST circuit, wherein tips of the pistons are opened and the surface of the control arm abutting the tips of the pistons are formed with grooves so that the tip opening of each of the pistons communicates with each of the grooves until the control arm rotated from its neutral position reaches a certain angle in the vicinity of the neutral position. Consequently, when the control arm is located in the vicinity of its neutral position, the oil generating a remaining hydraulic pressure in the HST circuit is drained through the groove, thereby suppressing the shock when the vehicle stops and expanding the neutral zone of the HST so as to securely hold the vehicle stationary. On the other hand, since the tip openings of the pistons are closed by the control arm when the control arm is rotated from the vicinity of its neutral position, i.e., the expanded neutral zone, the oil in the HST circuit is not drained so as to secure the good operational efficiency of the HST. The proposed means have these advantages.
For the disposal of the hydraulic pump and the hydraulic motor in the housing serving as the oil sump, the voluminal increase of hydraulic oil in the housing, which is caused by variation of the oil temperature, must be absorbed. Generally, an oil reservoir is provided for this absorption. A tank disposed outside the housing may serve as the oil reservoir. Otherwise, a part of an inner wall of the housing may be formed into an oil reservoir.
However, the arrangement of the tank as the oil reservoir outside the housing increases the number of parts and prevents the HST apparatus from being minimized. The oil reservoir formed of the housing requires a complicated mold for forming the housing, thereby increasing the number of processes and costs.
For solving the problems, it may be proposed that, instead of an oil reservoir, an amount of the hydraulic oil in the housing is reduced so as to be received therein even when it is expanded by heating. However, in the housing, the level of the reduced hydraulic oil is lowered and the air is filled thereabove. Accordingly, if a vehicle is left on a slope with its HST in neutral and with its engine at idle, the level of hydraulic oil may become lower than the tip opening of the piston or the groove, thereby causing the air in the housing to be unexpectedly absorbed into the HST circuit through the groove and the piston.
The air mixed with the hydraulic oil in the HST circuit causes the HST to free wheel wherein it may happen that the vehicle unexpectedly descends the slope or the vehicle does not start while it ascends the slope.
For another problem, if the neutral position of the HST is inaccurate, the hydraulic pump may unexpectedly discharge a little amount of hydraulic oil even when the HST is located at its neutral position, thereby driving the hydraulic motor so as to unexpectedly move the vehicle slowly. Therefore, some means for regulating hydraulicoil is required to solve this problem. However, if such means constantly drains increased hydraulic pressure oil from the HST circuit, the transmittal efficiency of the HST is reduced while the hydraulic pressure in the closed HST circuit must be increased in response to the operation of the HST.
An object of the present invention is to provide an HST comprising a housing serving as a fluid sump in which a hydraulic pump and a hydraulic motor are fluidly connected with each other through a pair of fluid passages formed in a center section so as to constitute a closed hydraulic circuit (an HST circuit), wherein the HST circuit is optimally communicated with the fluid sump so as to regulate the hydraulic pressure therein in correspondence to various operational states, thereby expanding the neutral zone of the HST so as to secure the stationary state of a vehicle, resolving the dynamic brake when the vehicle stops, and so on. The HST is provided with an output regulating device for adjusting a degree of reciprocation of pistons in a cylinder block of either the hydraulic pump or the hydraulic motor, wherein the output regulating device is interlockingly connected to a control shaft so as to be moved to a plurality of operational positions from a neutral position according to rotation of the control shaft so that fluid discharged from the hydraulic pump is made to flow into the hydraulic motor through one of the pair of fluid passages and is returned to the hydraulic pump through the other of the pair of fluid passages.
To achieve the object, according to the present invention, the pair of fluid passages are provided with respective opening portions on the same surface of the center section. A pair of pistons, which allow fluid to flow therein, are reciprocally inserted into the respective opening portions of the pair of fluid passages so as to be allowed to project outward from the outer surface of the center section and be retracted into the center section. A valve is provided for opening and closing the opening portions of the pair of fluid passages. A spring is extended between the valve and the pair of pistons for biasing the valve so as to close the valve. A member interlocking with said control shaft is provided so as to face the pair of pistons. When the output regulating device is located in the vicinity of the neutral position and fluid in a higher-pressured one of the pair of fluid passages forcedly opens the valve and flows into a corresponding one of the pair of pistons, the member introduces the fluid through an orifice into either the fluid sump or a lower-pressured one of the pair of fluid passages.
Accordingly, the control shaft is prevented from suddenly returning to its neutral position, thereby preventing a dynamic brake from being generated in the HST. When the output regulating device interlocking with the control shaft is located in the vicinity of its neutral position, fluid is drained from the higher-pressured fluid passage to the fluid sump or led to the lower-pressured fluid passage, thereby solving the remaining hydraulic pressure in the HST circuit. Thus, the vehicle is comfortably brought from its traveling state into its stationary state.
To achieve the object of the present invention, a pressure regulating device communicating with at least one of the pair of hydraulic fluid passages is interlockingly connected to a speed changing operation device so that, when the speed changing operation device is located within a certain range including its neutral position, the pressure regulating device is brought into a first pressure setting condition, and that, when the speed changing operation device is located out of the certain range, the pressure regulating device is brought into a second pressure setting condition.
When the pressure regulating device is put in the first pressure setting condition, the hydraulic pressure in the HST circuit is so low as to keep a vehicle having the HST stationary while the engine of the vehicle runs. When the pressure regulating device is put in the second pressure setting condition, the hydraulic pressure in the HST circuit is so high as to allow the vehicle to travel.
Accordingly, even if a hydraulic pressure difference occurs between the pair of fluid passages caused by an inaccurate neutral position of the HST while the speed changing operational device is located at its neutral position, the pressure regulating device is put into the first pressure setting condition so as to resolve the hydraulic pressure difference. In other words, the neutral zone of the HST is expanded so as to secure the stationary state of the vehicle. When the speed changing operation device is located out of the expanded neutral zone of the HST, the pressure regulating device is put into the second pressure setting condition so as to enable hydraulic pressure in one of the pair of fluid passages to be increased responsively according to the shift degree of the speed changing operation device.
The pressure regulating device comprises a relief valve for draining hydraulic fluid from the corresponding hydraulic fluid passage. A spring biases the relief valve so as to close it against the hydraulic pressure in the corresponding hydraulic fluid passage. The spring is retained by a spring-retainer. A movable arm interlocking with the speed changing operation device is provided with a contact surface. The spring biases the spring-retainer so as to abut against the contact plate.
For selectively establishing the first pressure setting condition and the second pressure setting condition by operation of the pressure regulating device, the spring-retainer is formed with an opening, and an escaping groove is formed on the contact surface of the movable arm so as to be open to the fluid sump. By operation of the speed changing operation device, the movable arm is moved so as to selectively open or close the opening of the spring-retainer to and from the escaping groove, thereby establishing either the first pressure setting condition or the second pressure setting condition. That is, the opening is opened to the escaping groove in the first pressure setting condition, and the opening is closed from the escaping groove in the second pressure setting condition.
Accordingly, the regulation of hydraulic pressure in the HST circuit can be performed by such a simple and inexpensive manner that the opening of the spring-retainer is opened/closed to or from the escaping groove so as to switch the pressure regulating device, thereby simplifying the HST and reducing the manufacturing costs thereof.
An amount of hydraulic fluid made to flow from each of the pair of hydraulic fluid passages to the escaping groove by the pressure regulating device may be limited by an orifice. Accordingly, the hydraulic pressure oil is prevented from being suddenly drained in a large amount from the HST circuit so as to prevent the HST from being lacking in hydraulic pressure oil. Also, the amount of hydraulic pressure in the HST circuit required to hold a vehicle in stationary on a slope can be maintained.
Alternatively, the pressure regulating device comprising the relief valve, the spring, the spring-retainer and the movable arm having the contact surface may be so constructed that the movable arm is provided with a projection projecting from the contact surface of the movable arm so as to push the spring-retainer and compress the spring. By operation of the speed changing operation device, the movable arm is moved so as to extend or compress the spring, thereby establishing either the first pressure setting condition or the second pressure setting condition. That is, in the first pressure setting condition, the spring-retainer is offset from the projection so as to extend the spring, thereby reducing a hydraulic pressure required to open the relief valve. In the second pressure setting condition, the spring-retainer is pushed by the projection so as to compress the spring, thereby increasing the hydraulic pressure required to open the relief valve.
Accordingly, the regulation of hydraulic pressure in the HST circuit can be performed by such a simple and inexpensive manner that the spring biasing the relief valve is extended/compressed by the projection and the spring-retainer so as to adjust the relief hydraulic pressure which is required to open the relief valve, thereby simplifying the HST and reducing the manufacturing costs thereof.
The pressure regulating device may be provided with a hydraulic bypass allowing hydraulic fluid to flow from higher-pressured one of said pair of hydraulic fluid passages to the other lower-pressured one of said pair of hydraulic fluid passages. An orifice is installed in said hydraulic bypass.
In comparison with the above mentioned pressure regulating device which drains hydraulic oil from the higher-pressured fluid passage to the fluid sump, the hydraulic bypass makes the HST circuit resistant to the shortage of hydraulic fluid.
Alternatively, instead of the orifice, the pressure regulating device having the hydraulic bypass may be provided with a bypass valve installed in the hydraulic bypass. The bypass valve is opened when the hydraulic pressure upstream thereof is less than a predetermined value, and closed when the hydraulic pressure upstream thereof is not less than the predetermined value. The predetermined value is less than the hydraulic pressure which is established when the pressure regulating device is put in the first pressure setting condition.
Accordingly, even if the hydraulic pump discharges an extremely small amount of hydraulic fluid to one of the pair of hydraulic fluid passages with the speed changing operation device located at its neutral position, because of an inaccuracy in the neutral position of the HST, the hydraulic pressure of the discharged fluid is lower than the predetermined value so that the bypass valve is still opened so as to bypass the discharged fluid to the other hydraulic fluid passage, thereby maintaining the vehicle in stationary. On the other hand, when the speed changing device is shifted for traveling of the vehicle, the hydraulic pressure upstream of the bypass valve is increased to be not less than the predetermined value so that the bypass valve becomes hard to be opened, thereby securing sufficient hydraulic pressure for driving the HST.
Furthermore, due to the bypass valve which is closed by the predetermined value of hydraulic pressure lower than that established in the first pressure setting condition, the hydraulic pressure in the HST circuit can be adjusted among three conditions, i.e., the non-pressurized condition established by opening the bypass valve, and the first and second pressure setting conditions which are established when the bypass valve is closed. Thus, the condition of hydraulic pressure in the HST circuit can be adjusted delicately in correspondence to various situations.
Other and further objects, features and advantages of the invention will appear more fully from the following description.